How does it compare to MongoDB, CouchDB and other "big" NoSQL databases ?

It does not compare. Different purposes, different design, different use case. For some projects it makes sense to use CodernityDB instead of MongoDB, CouchDB and others, for some other not.

Why Python 3 is not supported ?

Python 3 introduced many incompatible changes. In case of CodernityDB having working version for 2.x and 3.x series in the same codebase without ugly hacks (big monkey-patching etc.) is almost impossible. If you're interested Python 3 version of CodernityDB contact us. Porting CodernityDB to Python 3.x is not hard. Python 3.x support in fact was never needed. That's why there is no support for it (yet?).